The present invention relates generally to digital to analog converters and more specifically to an improved Gray code digital to analog converter.
The most common form of electrical digital to analog (D/A) converter is the switched R-2R ladder. It is simple and relatively inexpensive, consisting of a fixed reference voltage and for each ladder stage, providing conversion for each bit of the input digital word, two resistors and a single-pole, double-throw switch. A typical R-2R ladder D/A converter is shown in FIG. 1. Here, each switch switches between fixed logic "0" voltage V.sub.0 and logic "1" voltage V.sub.1 and by using series and shunt resistors R and 2R, and proper termination at the LSB to a reference voltage "V.sub.BIAS ", the ladder appears infinite in length. For any inputted binary code, an output voltage is developed at the MSB whose voltage level is related to the applied digital code by mapping the voltage level between V.sub.0 and V.sub.1 to the code words from all 0's to all 1's. A three stage ladder map is illustrated in FIG. 2.
This ladder will theoretically resolve code words of any length, but in practice is limited by the available precision of the ladder resistors. Ladder errors frequently are manifested by a condition known as non-monotonicity. If a code converter is monotonic, it's output will always increase for increasing values of input code, and vice versa. This condition is not always met with the binary code converters since it depends on the R-2R ratios of the ladder resistors being held to a precision proportional to the code convertor's quantization. For longer input words, more resistor precision is required.
A second problem with the binary R-2R ladder is the fact that it is binary weighted. Multiple switches must change state simultaneously for transitions between output states. For instance, when transiting midscale, the switches change from the MSB equal to 0 and all others equal to 1, to the MSB equal to 1 and all others equal to 0. Here, all switches change state. The switches themselves contribute electrical noise to the D/A converter output, and more noise is generated when more switches change state. These two effects, non-monoticity and ladder switching noise, limit the ultimate length of the R-2R converter.
This aspect of monotonicity is important, for in many applications resolution, and reduction in quantization noise, is more important than accuracy. A code converter which is monotonic will be more desirable for this type of application, since frequently the small-signal performance is of importance. An example is the emerging digital sound recording systems, in which an inexpensive coder converter with resolution to 14-16 bits are required to reproduce low-level passages. Low cost is important, for the intended consumer market. For a monotonic conventional binary coder, a 14-16 bit requirement could impose expensive requirements on resistor accuracy. Binary ladder switching noise is also a major drawback in this application for these code converters always operate with a signal centered at midscale, where the switching noise is maximum.
Thus, there is a need for a D/A ladder configuration, which should be inherently monotonic for any length, independent of ladder resistor accuracy. The network accuracy should only impact linearity, since overall linearity becomes of reduced importance in many applications, and the impact diminishes as signal level reduces.
These and other objects of the invention are attained by providing a digital to analog ladder configuration which is a Gray code converter to reduce switching noise and the switching points or voltages are within the ladder network and vary with the state of the ladder. In one configuration, the ladder is inherently monotonic and in another configuration the hardware required is minimized although no longer inherently monotonic. The inherently monotonic configuration includes for each stage a resistive network having first and second end terminals and a third terminal intermediate the first and second terminals. The first end terminal is connected to the third terminal of the previous stage and a switch selectively interconnects the second end terminal to either the second end terminal or third intermediate terminal of the preceding stage resistive network. The output terminal is connected to the third terminal of the last stage resistive network which is the least significant bit (LSB). The resistive values of the stages must be scaled in impedance or buffers may be used to prevent the resistive network of one stage from loading the previous stage. For the non-monotonic configuration, each resistive stage includes a switch which connects its first and second end terminals to either the second or third or third and second terminals, respectively of the preceding stage. The third terminal is selected such that it divides the resistive network into R and 2R resistances. The last stage from which the output is taken and which is the least significant bit has a resistive value of 2R and the third terminal of this last stage divides the resistors into a 0.5R and 1.5R resistances. In both configurations, the digital-to-analog converter is a Gray code converter. This reduces switching noises since only one switch at a time is changed for each step in the digital count.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.